Buck converters are widely used for DC-DC conversion, and preferably have high efficiency. To achieve high efficiency, the high-side and low-side power transistors of the buck converter output stage must be switched very fast and use as little margin (i.e. dead time) as possible. However, when switching very fast, a voltage is induced on the gate of at least one of the power transistors. Such an induced gate voltage leads to a turn-on of the device, cross-conduction and very high losses unless the dead time is very long.
Buck converters are conventionally packaged with the power transistors of the output stage disposed in one die (chip) and the driver on a separate die, or both the power transistors and the driver fully integrated on a single die. In the case of separate dies, the inductance between the driver and the gate of the power transistor is so high that the gate voltage cannot be perfectly controlled due to the dynamic voltage drop over the series parasitic resistance and inductance. This in turn induces a voltage at the gate of at least one of the power transistors unless the dead time is increased, resulting in reduced efficiency.
In the fully integrated case, the power transistors of the output stage are integrated with the driver on a single die. With this approach, the design of the power transistors is limited to the driver technology, which offers limited breakdown voltage. The maximum blocking voltage of the low-side and high-side transistors of the buck converter output stage are limited with such an approach. For example, Rdson (on-state resistance) and FOM (figure of merit) of the power transistors is very bad since only lateral transistors are available with conventional fully integrated approaches. The maximum efficiency of the fully integrated technique is therefore also limited.